There are numerous examples of devices which have a man-machine interface mounted on a fascia panel. For example, domestic appliances such as ovens and washing machines usually have a fascia panel which has controls which allow the user to set control parameters.
Capacitive sensors have been used in fascia panels for industrial ovens. Capacitive sensors are non-contact sensors, and are therefore less prone to mechanical wear than contact sensors. However, a disadvantage of capacitive sensors is that exposure to liquids such as water can lead to incorrect readings. It will be appreciated that ovens are often operated in a humid environment, and therefore water may condense on the fascia panel of the oven. It is therefore possible for a drop of water to form adjacent to a capacitive sensor causing an incorrect control parameter to be entered.
Inductive sensors are a known alternative to capacitive sensors. Typically, in an inductive position sensor the electromagnetic coupling between a transmit aerial and a receive aerial is varied in response to relative movement between a first member and a second member, thereby allowing the relative position of the first and second members to be determined from the signal induced in the receive aerial when an excitation signal is applied to the transmit aerial. For example, international patent application WO 03/038379 describes a position sensor in which a transmit aerial, formed by two excitation windings, and a receive aerial, formed by a sensor winding, are formed on a printed circuit board, and a resonant circuit is formed on a sensor element which is movable relative to the printed circuit board. The two excitation windings are shaped so that the electromagnetic coupling between the excitation windings and the resonant circuit varies along a measurement path, which is parallel with the plane of the printed circuit board, in accordance with a sine function and a cosine function respectively. By respectively applying an in-phase oscillating signal and a quadrature oscillating signal (that is 90° out of phase with the in-phase oscillating signal) to the two excitation windings, an oscillating signal is generated in the resonant circuit whose phase is dependent upon the position of the sensor element along the measurement path. The oscillating signal in the resonant circuit in turn induces an oscillating signal in the sensor winding whose phase is indicative of the position of the sensor element along the measurement path.
For the position sensor described in WO 03/038379, using conventional techniques it is difficult to deposit the two excitation windings so that the period of the associated sine and cosine functions is less than a few centimeters, and this limits the available resolution for position measurement. Further, the position sensor described in WO 03/038379 measures movement of a sensor element along a measurement path parallel to the plane of a planar member, and therefore is not well suited to applications in which movement of a sensor element transverse (i.e. crossing) to the plane of a planar member, for example a fascia panel, is to be measured.